Cathode ray oscilloscope



R. H. VARIAN CATHODE RAY OSCILLOSCOPE April 11, 1939-.

Filed March 15, 1935 RUSSELL H. VAR/4N.

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Patented A r. 11, 1939 umTE-o STATES PATENT OFFICE CATHODE RAYosolLLosoorn Russell H. Varian, San Francisco, Calif., assignor,

by mesne assignments, to Farnsworth Television & Radio Corporation,Dover, DeL, a corporation oi! Delaware Application March 13, 1935,Serial No. 10,888

a Glaims. (or. 250- 157) My invention relates to oscilloscope apparatus,and particularly to electrode construction for tubes used in suchapparatus, in combination with means for focusing the cathode ray beam.

Among the objects of my invention are: to

provide a cathode ray tube possessing greater sensitivity than is foundin tubes of like character in present use; to provide a tube which givesa greater degree of illumination at the screen thereof; to providemeans, in such a tube, adapted for use in a television receivingsystem,for the production of a cathode ray beam having a cross sectionof predetermined shape; to provide, in the type of tube described, amore positive grid control for the beam, and one whichhas a. sharpercut-off; to provide a cathode ray tube in which operationaldiiliculties, due to residual gas in the tube and to positive ionbombardment of the cathode, have been reduced to a minimum; to

provide a cathode ray tube construction in which the careful al-inementof tube elements is not essential to the eflicient operation of thedevice; to provide focusing means for the cathode ray beam which,together with animproved emitter, produces a spot on the tube screenhaving sharper definition; to provide an improved and simplifiedconstruction for cathode ray tubes; and to provide a simple method ofregulating the size of the luminescent spot in a cathode ray tube;

Other objectsof my invention will be apparent or will be specificallypointed out in the description forming a part of this specification, butI do not limit myself to the embodiment of the invention hereindescribed, as various forms may be adopted within the scope of theclaims.

Referring to the drawing:

Figure 1 is a vertical sectional elevation of a cathode ray tube andassociated apparatus incorporating the improvements of my invention.

Figure 2 is a plan view of one type of electron emitter adapted to .beused in the tube of my invention.

Fig. 3 is a side elevation of the electron emitter J Figure 8 is avertical sectional view of a modified accelerating electrode, showingits cooperative relationship with others of the electrodes.

Figure 9 is a modified form of electron emitter.

In broad terms, the cathode ray oscilloscope of my invention comprisesan evacuated envelope having a plurality of cooperating electrodesmounted therein. One of these electrodes is a cathode, preferablycomprising a main body portion, or heater, which is a comparatively pooremitter of electrons, and a smaller treatedportion which is highlyemissive, so that when the heateris energized a dense localized electrondischarge will be liberated from the treated portion of the cathode.

A second electrode, or accelerating anode, preferably in the form of ascreen, is provided, spaced from the cathode and provided with apositive charge, so that an intense electrostatic field is establishedbetween the two electrodes; This field causes the electron dischargefrom the cathode to be accelerated, at high velocity, in the formof abeam. The beam, due to its acquired velocity, passes through thepervious accelerating anode and is projected on a fluorescent screen,provided at one end of the envelope on the interior surface thereof.

Since the electrons in the beam normally tend to follow divergent paths,after escaping the electrostatic field, I prefer' to provide means, inthe absence of gas, for focusing the beam so that the area illuminatedby the beam, at the fluorescent screen, is of substantially the samearea as that area at the cathode from which the electrons were emitted.

This focusing means preferably comprises a solenoid disposed about theenvelope in axial alinement with the electron flow, so that energizationof the solenoid from a direct current source will establish a steadyelectromagnetic field along which the electrons are projected.

, This field prevents the electrons from following divergent paths, withthe result that with proper adjustment of focusing current, the spotilluminated thereby at the fluorescent'screen is a sharp definition ofthe area at the cathode from which the electrons were emitted.

, A third electrode, or control electrode, is proprovided closelyadjacent the cathode and disposed between the latter and theaccelerating anode. This control electrode, when provided with a varyingcharge, which may be caused, for example, by a signal train from theoutput of a television receiver, causes variations in the electrostaticfield between the cathode and the accelerating anode, with the resultthat-the electron flow from the cathode to the screen is proportional tothe charge on the control electrode. This variation of electron flowcauses fluctuations in brilliancy of the illuminated spot projected onthe fluorescent screen.

Means are preferably provided for deflecing the beam cyclically, in twodirections, so that an area of the fluorescent screen will be scanned insuccessive elementary portions, and these means, together with thevariations of spot illumination, caused by the control anode, willcreate an optical image on the screen, in accordance with the signalcurrent train delivered to the control electrode.

Two sets of beam deflecting coils are therefore provided, disposed aboutthe envelope so that their axes lie in a plane at right angles to theelectron flow, and the coils of each set thereof, disposed diametricallyopposite, are electrically connected.

An oscillator, preferably capable of delivering a current having asaw-tooth wave form, is connected to each set of coils and theseoscillators are adjusted to frequencies so that the fields of thedeflecting coils will cause the electron beam to be deflected cyclicallyto cause it to traverse the fluorescent screen in successive passes,both in vertical and horizontal directions.

In the present types of cathode ray tubes, for use in Oscilloscopes, aplurality of cooperating electrodes are provided, grouped at one end ofthe tube, comprising a cathode adapted to institute an electrondischarge, a focusing electrode, in the nature of a metallic cylinderdisposed in front of the cathode in axial alinement therewith, and ananode, spaced from the focusing electrode, comprising a plate having anaperture therein in alinement with the axis of the focusing electrode.

The function of the cathode is to institute an electron discharge whichis compressed into a beam by an electrostatic field created by anegative charge on the focusing electrode, and this beam is acceleratedtoward the anode, due to an attractive force exerted thereby on the beamcaused by a highly positive charge on this electrode.

Some of the electrons in the beam, due to their velocity, will passthrough the aperture and proceed to the. fluorescent screen; thequantity of electrons passing through the aperture being governed by thesize thereof and the compressed diameter of the beam. A large percentageof the emitted electrons, however, will not reach the screen, beingintercepted by the anode, with the result that the possible brilliancyofthe illuminated spot on the screen is greatly reduced.

There is still another factor which seriously aflects the brilliancy ofthe spot. It will be seen that if the anode aperture is not exactlyalined with the axis of the focusing electrode, the center of the beamwill not coincide with the aperture and, as a result, a still greaterquantity of the electrons will be intercepted by the anode and beprevented from reaching the screen.

Aside from these factors, still another detrimental feature presentsitself, in that as soon as the electrons pass through the aperture, theypass out of the influence of the focusing field with the result thatthey tend to follow divergent paths and arrive at the screen in greaterspaced relationship than when they passed through the aperture. Thisdiflusion of the electrons causes an illuminated spot on the screenwhich is blurred, such a spot being wholly unsuited for use intelevision receiving tubes where the sharpness of detail 0! portions ofthe reproduced image depends on the sharpness of outline of the spot,necessitating the use of additional focusing means.

I have provided an improved cathode ray tube, for uses of the characterdescribed, in which the majority of the electrons emitted by the cathodereach the screen and contribute to the brilliancy of illumination of thespot.

I have also provided means whereby the cathode may emit a beam ofrectangular cross-section, which feature is very desirable when the tubeis used to reproduce images in a television receiving system.

I have also provided means for focusing the electron beam so that theelectrons are prevented from diverging in their path from the cathode tothe screen, with the result that the spot on the screen is a sharpreproduction of the cathode area emitting the electrons.

In greater detail, my invention comprises an evacuated vitreous envelope2, provided with a domed end 3, which has a layer of willemite or otherfluorescent material deposited on the interior surface thereof, toprovide a fluorescent screen 4. The envelope is provided, at the endthereof opposite the screen, with a stem 6 having a press 7 forsupporting the leads of the several electrodes contained within theenvelope.

One of these electrodes is a cathode adapted to emit a localizedelectron discharge and, as shown in Figure 2, comprises a ribbon heatingelement 8, supported by the divergent ends of the leads 9 and adapted tobe excited by the battery II, as shown in Figure 4:. This element isconstructed of a'material which has a very low rate of electronemission, such as nickel. Provision is made for localizing the electrondischarge from the cathode by the disposition of a small deposit i2, ofan alkaline earth oxide, or a similar sub stance which is capable ofcopious electron emission, at a low heat, on the ribbon ii. It isobvious that if the temperature of the heater is maintained below apoint which causes little or no electron flow from the ribbon, theemission will occur substantially only from the deposit i2. Since suchis the case, it is also obvious that the size and shape of the depositgoverns the size of the electron beam as it leaves the cathode.

A second screen electrode, or accelerating anode I3, is provided, spacedfrom the cathode, and is supported by a ring i4 carried by the lead 56and the dummy l1 secured in the press 1. When this anode is highlycharged positively, with respect to the cathode, by the battery is, anintense electrostatic field is created between the electrodes whichresults in the electrons emitted by the cathode being accelerated in abeam l9 toward the anode. when they reach the anode they pass throughand continue on to the fluorescent screen.

Since the electrons in the beam will tend to follow divergent paths, assoon as they escape the influence of the electrostatic acceleratingfield, I provide means for restraining the electrons from following suchpaths, so that they will arrive at the screen within an areacorresponding to the area at the cathode from which they were emitted.Disposed about the envelope is a solenoid 2| which is supplied withcurrent from a. direct current source 22 through a variable resistor 23.Energization oi the solenoid will create a powerful. magnetic fieldsubstantially parallel.

to the electron flow, with the result that the tendency of the electronsto deviate from parill allel paths is controlled. By proper adjustmentof the current through the focusing solenoid 2i, the electrons arefocused in the plane of the screen, thus giving rise to a light spot ofthe size and shape of the emitting area.

While the foregoing description deals with focusing means for preventionof diversion of the electrons in the beam, 1 may prefer to use anon-divergent accelerating field which would produce a practicallynon-divergent beam without the aid of the focusing means. This fieldcould be produced in a number of ways; for example, by providing acylindrical electrode about the path of the beam and charging theelectrode to create a uniform electrostatic field through which theelectrons of the beam travel on their' way to the screen. The fieldeffectively prevents divergence of the electrons from parallel paths,and assures their arrival at the screen within an area corresponding tothe defined area from which they were emitted at the cathode.

Means are provided for deflecting the beam I9 cyclically so that it willtraverse an area of the fluorescent screen in successive passes, both inhorizontal and vertical directions. Beam deflecting coils 24 and 26,shown diagrammatically in Figure 1', are provided and are disposed inpairs about the envelope so that their axes lie at right angles to theelectron flow. Each pair of coils is connected to oscillators 21 and 28,respectively, which are capable of producing an alternating outputcurrent having a saw-tooth wave form. The alternating fields of thesecoils will cause the beam to be deflected so that it will traversesuccessive linear portions of the fluorescent screen, having a widthdepending on the diameter of the beam. 1 A- screen electrode, or controlelectrode 29, is provided between the accelerating anode and thecathode, and is supported by a ring 3! which-is carried by a lead 32 anda dummy 33 secured in the press I. The lead 32 is connected with a tap34 of the battery l8, and this places a charge'on the anode 29 which isslightly positive with respect to the cathode, and negative with respectto the accelerating anode.

By connecting the electrode 29 through a blocking condenser 36 and alead .31, with the output of apparatus, which may be, for example, areceiver of television signals, and connecting the return circuit fromthe apparatus to the negative-terminal oi the battery li-by alead 38,the varying amplitudes of the apparatus output current applied to thecontrol electrode will cause modifications oi the electrostatic field,existing between the accelerating anode and the cathode, with ther'esult that the quantity of electrons allowed to pass the electrode 29,at a given instant; is proportional to the charge on the controlelectrode caused by the amplitude of the signal at-that instant. Thisvariation of electron flow causes fluctuations in the brilliancy ofillumination of the spot at the screen I, and, in

combination with the scanning action, will produce an image on thescreen which is an optical translation of the train of signals.

In Figure 3 I have illustrated a cathode construction by means of whichan electron beam, having a rectangular cross section, may be produced.This cathode comprises a pair of ribbons 39 and ll which together makeup the heater.

The ribbon 39 is provided with a rectangular aperture l2, and a thinthoriated or oxide coated plate 43 is disposed between the ribbons sothat a portion thereof is exposed by the aperture.

and the least This exposed portion is the surface from which theelectrons are emitted, and it will be seen that the electron beam, as itleaves the cathode, will have a cross section governed by the shape ofthe aperture 42. This rectangular beam produces a spot of correspondingshape at the screen, and produces a completed television image ofgreater brilliancy and sharpness than one produced by a .beam ofcircular cross section, as greater coverage ofthe screen area isachieved. It will be readily seen that the electron distribution of abeam of circular cross section, taken in successive planes from thecenter to the periphery thereof, with respect to the direction of travelof the beam, decreases as the chordal distance decreases, hence thegreatest illumination of a path traversed by such a beam will be locatedat the center thereof amount of illumination will be obtained at theedge of thepath. This sparcity of electrons produces the familiar linearshadow areas observed between the lines of television images. By theuseof a beam of rectangular cross section, the electron distribution inany plane parallel with the path of beam travel isuniform, and byadjusting the deflecting oscillators to move the beam, in steps notgreater than the thickness thereof, it will be seen that there [will beno visible line of demarcation between successive passes of the beam andconsequently the reproduced image will be sharper and possessed of agreater degree of brilliancy.

Figure '7 shows still another modified form of cathode in which theoxide deposit 44 is disposed on the surface of the heater ribbon 46 awayfrom the fluorescent screen. An aperture 41, which may be eitherrectangular or circular in form, is provided in' the ribbon. I havefound that electrons emitted by the oxide deposit 44 will be drawnthrough the aperture 41 to form the beam. It will be understood that thecross sectional shape of the beam will depend on the shape of theaperture 41. r t

Figure 5 illustrates a modified construction for the control electrodeand accelerating anode. The electrodes illustrated in Figure 1 areconstructed of fine wire mesh, and consequently, due to the interceptionof a certain percentage of the electrons by the wires, due to theirbeing positioned in the path of electron flow, the possible brilliancyof the spot at the screen will be slightly reduced. Although theelectrodes formed of fine mesh screen provide a trifle better operationof the tube, the construction shown in Figure 5 will give substantiallyas good results in addition to greatly reducing the electroninterception by these electrodes. Each of the supporting rings l4 and 3|are provided with a plurality of fine spaced wires 43, those of thecontrol anode being disposed at right angles to those of theaccelerating anode. These wires may be spaced apart a greater distancethan those of the wire mesh without noticeably affecting the efficientfunctioning of the electrodes. It will be seen that since there thepyramid formed by the cyclically deflecting electron beam. This type ofanode, when charged, will produce an electrostatic fleld which willprovide satisfactory operation of the tube and will provide thedesirable feature of not intercepting any of the electrons in the beam.

In conclusion, the cathode ray oscilloscope just described ofl'ersdistinct improvements over like devices in present use by providingsuperior cathode constructions, which permit substantially all of theelectrons emitted thereby to reach the fluorescent screen andconsequently produce an illuminated spot of maximum briiliancy; byeliminating the apertured anode which permits only a small portion ofthe emitted electrons to reach the screen; by providing electrodeconstruction which permits positioning of the control anode closelyadjacent the cathode, thereby greatly increasing the mutual conductanceof the tube and permitting complete control'and sharper cut-off of theelectron flow with very low electrode potentials; by providing amagnetic focusing field through which the electrons fiow,-therebypermitting sharper definition of the spot on the fluorescent screen; byproviding cathode constructions which permit the construction of cathoderay tube in which the electron beam is possessed of a crosssection ofpredetermined shape; by providing a cathode ray tube which does notrequire the accurate alinement of cooperating electrodes to insureefllcient operation thereof; by providing a cathode ray tube whereinionization, due to electron collision, is reducedto a minimum due to thesmall quantity of oxide used as the emitting element and the fact thatthe majority of emitted electrons reach the screen; by providing asimplifled and easily constructed cathode ray tube; and by providingcathode constructions, for use in cathode ray tubes, utilizing gas asthe focusing medium, which, due to the small quantity of oxide usedtherewith, are not seriously affected by positive ion bombardment of thecathode caused by the gas.

While I have chosen to illustrate and describe the improvements of myinvention in conjunction with a cathode ray tube and a televisionreceiving system, I do not desire to be limited by such illustration anddescription, as uses for my improvements, in other electronic or spacedischarge devices, will readily be apparent to those skilled in the art.

I claim:

1. A'cathode ray tube comprising an, envelope containing a cathodesupport member capable of being heated and having a substantially flatsurface of substantially non-electron emitting material, an area ofthermionic electron emitting material deposited on said flat surface andheatable thereby to electron emitting temperature, said area being'ofelemental dimensions, a planar accelerating screen having multipleapertures therein positioned parallel with and close to said flatsurface, and having an area substantially larger than the area of .saidemitting material, a fluorescent screen of picture size in the path ofelectrons passing through said accelerating screen, a magnetic solenoidsurrounding the entire path between said emitting material and saidfluorescent screen, and. scanning means positioned to act on electronsafter they leave said accelerating screen and before they arrive at saidfluorescent screen.

2. A cathode ray tube in accordance with claim 1, with a planar gridbetween said accelerating screen and said emitting material, said gridbeing of substantially greater area than said emitting material andpositioned parallel to both said accelerating screen and said surface.

3. A cathode ray tube in accordance with claim 1, with a planar gridbetween said accelerating screen and said emitting material, said gridbeing of substantially greater area than said emitting material andpositioned parallel to both said accelerating screen and said surface,and wherein the accelerating screen is made up of equally spacedstraight wires extending in one direction only, with the grid made up ofequally spaced straight wires extending only in a directionsubstantially at right angles to the wires of said accelerating screen.

4. A cathode ray tube having an envelope containing a fluorescent screenat one end thereof, a thermionic cathode at the other end thereof and afiat screen accelerating electrode between said cathode and saidfluorescent screen, said cathode comprising a thin flat sheet ofnon-emitting material adjacent and parallel to said accelerating screenand having an aperture therein of elemental dimensions, and a layer ofthermionic electron emitting material confined solely to the backsurface of said sheet away from said accelerating screen and adjacentthe edges of said aperture, the side walls of said aperture and thefront surface of said sheet being free from electron emittin'g material.

RUSSELL H. VARIAN.

